Full length and alternatively spliced pgp1 transcripts in multidrug-resistant Chinese hamster lung cells

In an effort to better understand the preferential resistance to actinomycin D displayed by the multidrug-resistant Chinese hamster lung cell line DC-3F/ADX, we have cloned from those cells a number of cDNAs representing p-glycoprotein gene transcripts. Of the 12 clones isolated, all represent pgp1 transcripts and one, pADX165, contains a 4304-base pair insert with an open reading frame encoding a 1276-amino acid protein that is the homolog of the mouse mdr3/mdr1a gene product. A domain by domain comparison of this protein with p-glycoproteins capable of supporting multidrug resistance, i.e. human mdr1, mouse mdr1/mdr1b, and mouse mdr3/mdr1a, shows that, in addition to the ATP binding sites, the second, fourth, and eleventh transmembrane domains and the four small intracellular loops, IC-1, IC-2, IC-4, and IC-5, are highly conserved and are therefore likely to be important for the maintenance of p-glycoprotein function. Of the remaining 11 cDNA clones, 9 were found to be truncated versions of pADX165. Two others, however, pADX185 and pADX124, contained internal deletions resulting in open reading frames capable of encoding lnovel forms of p-glycoprotein. S1 nuclease and RNase protection analysis demonstrated that these cDNAs represent transcripts present in a number of different multidrug-resistant Chinese hamster lung cell lines. Hence, both are considered to be splicing variants of the hamster pgp1 gene primary transcript.